This invention relates to a pulse slimmer amplifier circuit for a magnetic disk drive. More particularly, this invention relates to compensating a differentiating amplifier for variations in the values of filter elements in the amplifier.
In magnetic recording systems, information is often recorded on a magnetic disk which is divided into concentric tracks. The concentric tracks, in turn, are divided into sectors for storing magnetically encoded information. A transducer, or read head, flies above the surface of the magnetic disk to magnetically encode information onto the disk or read magnetically encoded information from the disk. When reading information from the disk, the read head flies over selectively magnetized portions of the disk. The selectively magnetized portions represent digital information stored on the magnetic disk. The read head, when flying over the magnetized portions, generates a pulsed signal representative of the digital information stored on the disk. This pulsed signal is then amplified and demodulated to recover the digital information.
In certain types of magnetic recording schemes, digital information is recorded at different frequencies. The recording frequency is dependent upon the concentric track of the magnetic disk upon which the information is being written. For example, employing a technique called, "zone-density recording", information on different tracks is recorded at different frequencies. In zone-density recording, the recording frequency is varied to maintain a consistent bit density on the magnetic disk from the inside track to the outer most track. In the recovery of information which has been recorded using zone-density recording, it is necessary to employ a differential amplifier having a bandwidth capable of receiving the pulsed signal and detecting the frequencies of the data pulses. The recording frequency varies from track to track, and can vary as much as two (2) times over the radius of the disk.
As the recording frequency increases, the data pulses read from the magnetic disk, which represent the digital information stored on the magnetic disk, tend to crowd together and become much more difficult to separate in the reading and demodulating circuitry. Therefore, a technique known as pulse slimming is used in the recovery of the data. Pulse slimming makes the pulses read from the magnetic disk narrower or slimmer. This is accomplished at both the leading and trailing edges of the pulse by subtracting the first derivative of the pulse from the pulse itself.
The circuit elements used to obtain the first derivative of the data pulses retrieved from the magnetic disk are connected to form an integrated differentiator circuit. The differentiator includes a filter network to vary the pole position of the amplifier as the recording frequency varies. This is described in more detail in the patent application entitled ADJUSTABLE BANDWIDTH DIFFERENTIATING AMPLIFIER FOR MAGNETIC DISK DRIVE filed on Apr. 26, 1989 with Ser. No. 07/344,166, with common inventors and assigned to the same assignee as the present invention.
The filter circuit is typically an RC network which includes resistive and capacitive elements. As recording frequencies in the magnetic disk drive industry increase, the value of the filter components used in the differentiator generally decrease. Therefore, the required value of capacitive elements in the filter circuit of the differentiator can be required to be so small that external components cannot be used. In that case, the capacitive elements must be integrated onto a die containing the differentiator.
However, the value of an integrated capacitor can vary significantly with wafer processing. Since the RC product (i.e., a product of the resistive elements and the capacitive elements) in the filter in the differentiator determine the pole position of the differentiator, variations in the values of the capacitive elements cause an undesirable shift in the pole position. Thus, there is a need for a method and apparatus to compensate for variations of element values in an amplifier to maintain desired amplifier characteristics.